| The study on ultrastructure of tobacco leaves was conducted by using transmission electron microscope. Based on the analysis of physiological and biochemical characteristics, the principal changes were found as the followings.1 The losing of the order grana lamella in chloroplast appeared firstly and most obviously. The mitochondria and nuclear could keep their functions until late senescence stage. The plasma membrane was the last decomposed organdie. This highly ordinal decline could meet the need of cell metabolism for energy. During leaf maturity, the changes of chloroplast ultrastructure were observed to be the most evident, and the development of chloroplast was not synchronal, while the proplastid and chloroplast of different stages could be seen in one cell. The full development of starch grain contributed to the irregular chloroplast. The ratio of low grana lamella(<10%) to the total grana lamella in pre-maturity was 14.16%, but in over-maturity it reached 73.12%.2 Involving the definite changes of mitochondria ultrastructure, the amount of cristae was observed to decrease evidently, and the inner membrane was broken, while the area and intension of respiration reduced in the over-maturity stage.3 During the pre-maturity stage, mitochondria surrounded the chloroplast in anisomerous distribution. And it was observed that the amount on anear cell wall side was larger than on another side in pre-maturity instead of the late senescence stage, which showed the requirement of more energy to keep the physiological functions of the cell.4 In the late stage of maturity, the clusters of high electron-dense was found at chloroplast and mitochondria, also some peroxisome and the growing nuclear heterochromaty were observed, which indicates the programmed death of the cells.5 The physiological and biochemical analysis showed that chlorophyl was evidently affected by leaf senescence, and the destruction of chloroplast grana lamella could result in the complete cell senescence.
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